Effect of silicic acid on arsenate and arsenite retention mechanisms on 6-L ferrihydrite: A spectroscopic and batch adsorption approach.
Effect of silicic acid on arsenate and arsenite retention mechanisms on 6-L ferrihydrite: A spectroscopic and batch adsorption approach.
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DOI：
10.1016/j.apgeochem.2013.09.005
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发表时间：
2013-11
影响因子：
3.4
通讯作者：
Chorover, Jon
中科院分区：
文献类型：
--
作者：
Gao, Xiaodong;Root, Robert A.;Farrell, James;Ela, Wendell;Chorover, Jon
采用间歇吸附实验、衰减全反射傅立叶变换红外（ATR-FTIR）光谱、扩展x射线吸收精细结构（EXAFS）光谱和密度泛函数理论（DFT）模型，研究了砷酸盐和亚砷酸盐在铁水合物-水界面上在宽pH范围内的竞争吸附。间歇式吸附结果表明，砷酸盐和亚砷酸盐在6-L水合铁表面的吸附表现出很强的pH依赖性，且pH对砷酸盐和亚砷酸盐对砷的吸附影响不同。随着pH值的增加，砷酸盐的吸附量呈下降趋势；而砷酸盐的吸附随着pH值的增加而增加，在pH值为7 ~ 9时达到最大值，吸附后随着pH值的进一步增加而减少。结果表明，在实验条件下，硅酸和砷酸盐之间的竞争吸附可以忽略不计；然而，在低pH和高pH条件下，观察到硅酸和亚砷酸盐之间存在强烈的竞争吸附，特别是在低pH和高pH条件下。现场流动的ATR-FTIR数据显示，在没有硅酸的情况下，在整个pH范围内，砷酸盐在水合铁表面形成内球、双核双齿配合物。在本研究探测的整个pH范围内（pH 2.8 - 9.0），硅酸也在水合铁表面形成球内配合物。ATR-FTIR数据还显示，在所研究的环境相关浓度（例如1.0 mM）下，硅酸在水合铁表面发生聚合。根据ATR-FTIR数据，砷酸盐的络合方式不受硅酸存在的影响。EXAFS分析和DFT模型证实，砷酸盐四面体通过双核双齿络合与Fe金属中心成键，平均As(V)-Fe键距为3.27 Å。EXAFS数据表明，亚砷酸盐与6-L水合铁形成单核双齿和双核双齿配合物，as (III)-Fe键距分别为~2.92 ~ 2.94和3.41 ~ 3.44 Å。在Si的存在下，砷酸盐和亚砷酸盐的EXAFS光谱中As-Fe键距离保持不变，这表明尽管Si优先减少亚砷酸盐的吸附，但它对As-Fe键合机制的影响可以忽略不计。
The competitive adsorption of arsenate and arsenite with silicic acid at the ferrihydrite-water interface was investigated over a wide pH range using batch sorption experiments, attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy, extended X-ray absorption fine structure (EXAFS) spectroscopy, and density functional theory (DFT) modeling. Batch sorption results indicate that the adsorption of arsenate and arsenite on the 6-L ferrihydrite surface exhibits a strong pH-dependence, and the effect of pH on arsenic sorption differs between arsenate and arsenite. Arsenate adsorption decreases consistently with increasing pH; whereas arsenite adsorption initially increases with pH to a sorption maximum at pH 7–9, where after sorption decreases with further increases in pH. Results indicate that competitive adsorption between silicic acid and arsenate is negligible under the experimental conditions; whereas strong competitive adsorption was observed between silicic acid and arsenite, particularly at low and high pH. In-situ, flow-through ATR-FTIR data reveal that in the absence of silicic acid, arsenate forms inner-sphere, binuclear bidentate, complexes at the ferrihydrite surface across the entire pH range. Silicic acid also forms inner-sphere complexes at ferrihydrite surfaces throughout the entire pH range probed by this study (pH 2.8 – 9.0). The ATR-FTIR data also reveal that silicic acid undergoes polymerization at the ferrihydrite surface under the environmentally-relevant concentrations studied (e.g., 1.0 mM). According to ATR-FTIR data, arsenate complexation mode was not affected by the presence of silicic acid. EXAFS analyses and DFT modeling confirmed that arsenate tetrahedra were bonded to Fe metal centers via binuclear bidentate complexation with average As(V)-Fe bond distance of 3.27 Å. The EXAFS data indicate that arsenite forms both mononuclear bidentate and binuclear bidentate complexes with 6-L ferrihydrite as indicated by two As(III)-Fe bond distances of ~2.92–2.94 and 3.41–3.44 Å, respectively. The As-Fe bond distances in both arsenate and arsenite EXAFS spectra remained unchanged in the presence of Si, suggesting that whereas Si diminishes arsenite adsorption preferentially, it has a negligible effect on As-Fe bonding mechanisms.
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